Resizing Render window?

How can you set the size for render window? I like to set the window small for quick render check.

In your ‘render’ buttons panel, push the button that says ‘25%’ for the smallest render window.

Ya ok I found the buttons. But still, suppose you want to generate and save image size of 2000 x 3000 pixel. How can you set that?

Render Scene Panels (F10)
On the Format tab, set ‘size x’ and ‘size y’

Ok I will try that. Thanks.